As a developing device used in an image forming apparatus such as a copying machine or a printer, there is a device that performs development using a two-component developer. In the developing device that uses the two-component developer, a toner equivalent to an amount consumed by a development operation is supplied. However, in such a developing device, performance of a carrier falls and charging performance of the toner is deteriorated while the toner is supplied.
A system called trickle development system is provided in order to suppress the deterioration in the charging performance of the toner. The trickle development system is a system for supplying a new carrier (a concentrated toner) to a development container separately from the toner supplied to supplement the consumed amount. An excess developer that cannot be stored in the development container because of the supply of the carrier is discharged from a discharge port. In this way, the deteriorated carrier is replaced with the new carrier little by little.
As such a developing device of the trickle system, for example, JP-A-2000-81787 discloses a developing device that holds up a developer and then discharges the developer from a discharge port in which a developer scattering preventive member is arranged.
On the other hand, in the developing device of the trickle system, a device that detects toner density in a holdup position of a developer near a discharge port is provided. A certain amount of the developer needs to be accumulated in a detection position for a toner density sensor to measure magnetic permeability and detect toner density. However, when the surface of the toner density sensor is soiled, misdetection occurs. Therefore, there is a device that rotates a blade and sweeps the surface of the toner density sensor with the blade to remove the soil.
When the blade is rotated near the discharge port of such a device, the developer is likely to be further swelled by the blade in the holdup position of the developer. The developer is excessively discharged from the discharge port because of the further swell of the developer by the blade. Therefore, an amount of the discharge of the developer is not stabilized. This is likely to affect the feeding of the developer to a developing roller.
Therefore, even when toner density is detected by making use of the holdup of the developer for the discharge of the developer from the discharge port, the developer is stably discharged from the discharge port. As a result, there is a demand for the development of a developing device that can stabilize an amount of the developer in the development container and stably feed the developer to the developing roller.